Live fish caught at record depth

A live deep-sea fish has been caught at a record depth of 2,300m on the hot vents of the Mid-Atlantic Ridge.

Three shrimp species were also pulled to the surface, researchers report in the journal Deep-Sea Research.

Scientists have engineered a new device that allows recovery of live animals under their natural pressure at greater depths than previously achieved.

Next they hope to be able to transfer the animals into an experimental lab to study their normal biology.

"Pressurised recovery has been around for the past 30 years, but this is the deepest fish-capture under pressure - the previous record was 1,400m. This is also the first time pressurised capture has occurred at a hydrothermal vent," said Dr Bruce Shillito, marine biologist at the Universite Pierre et Marie Curie, Paris, France.

The shrimp species were caught at 1,700m (5,600ft; Mirocaris fortunata and Chorocaris chacei) and 2,300m (7,500ft; Rimicaris exoculata) at two vent fields, Lucky Strike and Rainbow, on the Mid-Atlantic Ridge.

Scientists watch the deep-sea fish inside the Periscop onboard ship

Dr Shillito explains: "At depths of over 1,000m, it is difficult to recover animals alive. Catching with no pressure is as good as catching dead. Fish are the most fragile - even a fisherman with a 100m line will probably reel in a catch whose gas bladder is in its mouth."

Although the fish caught by the team was a zoarcid (Pachycara saldanhai) and had no gas bladder, it was sensitive to full decompression.

At the surface, under pressure, the fish was active and remained upright, however upon release of pressure its movement became uncoordinated and within a few minutes it was totally motionless.

A similar effect, caused by decompression, was also observed in the shrimp species. At the surface, under pressure, most shrimps were in an upright position and swimming actively and continuously.

When a separate shrimp sample was caught and pulled to the surface without pressure, the animals jerked violently, and after a few hours were dead.

The samples were examined onboard the ship "Pourquoi Pas?" during the Momareto cruise, which was organised by Ifremer, the French Research Institute for Exploitation of the Sea.

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First deep-sea shrimps caught around hydrothermal vent

The next step for Dr Shillito's team is to be able to transfer its catch from the sampling device into a better equipped experimental tank, without decompression, allowing the scientists to observe the animals' normal behaviour and responses to different environments.

"We are particularly interested in the Pompeii worm (Alvinella pompejana), a vent worm which is thought to be the most thermo-tolerant marine organism, yet remains to be recovered in good enough shape. It is intriguing to find out how heat-resistant this animal is," Dr Shillito told BBC News.

Despite covering about 60% of the Earth's surface, the deep-sea floor ecosystem is poorly understood. Dr Shillito says: "We urgently need to find out more about the place we are destroying."

He adds: "At a time when we are over-fishing the depths of the ocean, we know more about cooking recipes than the biological features of deep-sea fauna."

A new device

The new sampling system for pressured recovery, which has been named Periscop, was developed by Dr Shillito in conjunction with Mr Gerard Hamel, a mechanics engineer at the Universite Pierre et Marie Curie.

It received funding from Exocet/d, a large European research programme.

It has three compartments which perform different tasks - capture at depth, recovery of the deep-sea species under natural pressure, and transfer to the lab with no decompression.

"In most previous attempts involving pressurised recovery, a single container fulfilled these three tasks - this may lead to contradictory technical requirements," explains Dr Shillito.

The plastic capture box is attached to a submersible arm which allows movement and suction for sampling. The animal is then transferred into a pressurised box. This is kept at the same pressure as the sampling depth during ascent by a pressure compensator.

"We used pressurised water to maintain pressure, which is a safer and a simpler alternative to gas. We hope this method of pressurised recovery will become standard," Dr Shillito said.

As well as pressure shock, when animals are pulled to the surface they suffer from changes in temperature.

"The temperature at depths below 2,000m is pretty constant all over the world - around 2-4C, yet the surface waters where we were sampling were 22-25C," explains Dr Shillito.

He adds: "Heating is difficult to prevent without getting out the heavy gear - using active cooling systems, requiring energy and computer controls - but at least we know that every sample has had the same temperature history; they have the same background story."

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